ISG5 - WG6 report

(3) Reports from ISG5 meeting: Tauchi briefly reported some topics which have been discussed at the ISG5 meeting at SLAC, 2/22-2/25,2000. The meeting has begun from a greeting by new SLAC director J.Dorfan. From KEK, T.Tauchi, H.Yamaoka, K.Yokoya and K.Kubo participated in the working group 6 (Interaction Region).

On Tuesday (2/22), there were two talks and a discussion with ISG report editors.

(a)The first talk was "Consumable collimator engineering status" by E.Doyle(SLAC). He described engineering studies on two types of the collimator which are rotary (circle) and tape. Both are kinds of scraper and their thickness is 1/4 radiation length(r.l.) to be optimized between 0.1 and 1 r.l. . The typical aperture is 200 - 2000 micron-m with the beam sizes of sigma_x/sigma_y=40/5micron-m there. The tape is titanium of 50 micron-m thickness and the main body of the rotary type is beryllium whose surface is covered by copper of 50 micron-m thickness. He estimated temperature rises; 283-degree_C at constant on the tape and 23 degree_C with 2 W at the rotary.

(b)The second talk was "Q1 (permanent magnet) support" by K.Skarpaas(SLAC). The Q1 is 14.29mm diameter and 2.1m long and it is supported in a tube of 50cm diameter. One of complexities is an existence of exit beam line, remembering 20mrad crossing. The Q1 itself must be isolated from outside world such as ground motion and artificial vibration as possible as we can. If a magnetic field excluder is necessary especially for the large detector option, it might be very difficult to install all structures inside the 50cm-phi tube.

On Wednesday (2/23), there are 6 talks and a joint session with WG1 (parameters) discussing on "pre-linac collimation".

(a) "e+e- pair background issues on NLC-IR" by J.Gronberg. He explained a new additional mask very near the IP, which is made of Beryllium. The purpose is to absorb low-energy secondary electrons/positrons which are back-scattered at the exit-beam line. With this mask, a hit density successfully decreased from 10 to 2 hits/mm^2/train at r=1.2cm (VTX-1st layer) for B=6 Tesla small detector option. He also compared two programs for neutron scattering and transportation, which are Fluka98 and COG. The former program has discreet scattering angle while the latter has continuous one. Both results were turned out to be consistent since the discreet angles might be smeared out with multiple scatterings. He also confirmed that the major neutrons were generated at the conical mask close to the IP.

(b) "Backgrounds from beam loss at the final focus system" by T.Maruyama. He studies beam losses along the final focus beam line after the collimation. The loss is caused by beam-gas elastic, inelastic and thermal photonic scatterings, that is Coulomb scattering, bremmstrahlung process and a scattering with thermal photons inside beam pipe, respectively, with a vacuum pressure to be 10^{-8} torr. All the processes have been simulated/estimated by DIMAD2 from the exit of linac to the entrance of the final focus system. Then, trackings of tail-particles are passed to GEANT for background estimation. Present results show that numbers of beam-loss particles are 1000, 500 and 51/train for elastic, inelastic and thermal processes, respectively. Background estimation in detectors is a future study.

(c) "Muon backgrounds in 5km BDS (NLC)" by L.Keller. He estimated muon backgrounds in the electromagnetic calorimeter (small detector) for three cases such as no muon shielding, muon spoiler and muon attenuator. The muon spoiler, which is so-called "tunnel filler" of 9m long, is located at 490m from the IP, while the muon attenuator of 54m long magnetized iron cylinders is located at 3035m from the IP. He calculated suppression factors of 1/100 and 1/10 for the spoiler and attenuator, respectively, compared to no shielding. He also found that "no shielding" can be acceptable at E_beam=250GeV though it is marginal. For higher beam energy, the muon attenuator is marginal. When an additional safety factor is necessary, the muon spoiler is the best choice.

(d) Pair monitor and active mask/luminosity studies (JLC) by T.Tauchi. The contents were already reported at the FFIR meetings, so please refer previous minutes. SLAC people have interesting in R&D of 3D-pixel device for the pair monitor.

(e) JLC QC1 support system by H.Yamaoka. This talk was also presented at the previous FFIR meeting.

(f) JLC support tube calculations by E.Doyle. He confirmed the JLC support tube calculations by using ANSYS program, which have been executed by H.Yamaoka. His new point was to make a more realistic model by taking account of imperfection of the support tube since symmetry of ideal model produces no differential motion. This work is very impressive since engineers of both KEK and SLAC were collaborating to analyze the same problem. Since they use the same program of ANSYS, detailed informations can be easily exchanged.

(g) JLC dump line design by K.Kubo. This was also already reported at this meeting. The major attracted point was a vertical bend to measure beam energy spread with O(0.1%) resolution. With the NLC's suppression factor by neutron shielding, that is 10^{-4}, the dump line can be designed at E_beam=250GeV. At higher energies, more studies must be needed.

"Discussion for pre-linac collimation" with WG1. The pre-linac collimation is located after 8 GeV pre-linac. The major question was " Is it possible to eliminate a long main-linac collimation with a perfect pre-linac collimation?". During discussions it became to be clear that the most difficult part must be longitudinal collimation (which may be within 5 sigma_z) in order to ensure +/- 2% momentum spread at the exit of main-linac. At NLC, probabilities of re-populations due to beam-gas scatterings in tails have been estimated to be 10^{-5} and 10^{-9} for pre-linac and main-linac collimations, respectively, assuming vacuum pressure of 10^{-8}torr. Therefore, 10^7 and 10^3 electrons may be loss per train in the BDS. Prompt answer was "No" given by T.Raubenheimer because of backgrounds and machine protection. The machine protection means that miss-fired beam should be over-focussed before hitting and destroying any optical elements including beam pipes. So, a next question was raised as "If the backgrounds are acceptable for detectors, how much will the collimation section be shortened?". The length may decrease significantly. Apparently the beta-section must be simplified, however the energy tail collimation needs more studies. As clearly mentioned above, the answer strongly depend on the vacuum pressure. If the pressure can decrease to 10^{10} torr, the answer may change. So, we requested such a low vacuum system in the main linac. Also, detector background studies are highly necessary for beam loss at the BDS.

On Thursday (2/24), there were two talks.

The first talk was "proposed new FF design" given by P.Raimondi. This talk was one of highlights at this ISG5 meeting. The new FF has features of very short length and large l* (distance from the IP). The length is about 1/10 of the NLC-FF, say ~200m and l* can be 4.24m. The key idea is a local chromaticity correction adding a sextuple between final doublets. So, the chromatic correction section (CCS) can be eliminated in FF system. A.Sery has checked this new FF optics by comparing to the NLC-FF optics. He found that the new design was good as the NLC-FF one. Possible drawback may be a tunability ( it may have complicated knobs ), which must be carefully analyzed. The longer l* is also attracted since the QC1's can be located outside of strong solenoid field and more space can be available in such busy area. It so fascinates that N.Yamamoto (KEK) is going to analyze this new FF optics for JLC-FF.

Next talk was "permanent magnet in BDS" by A.Ringwall. Originally, the NLC has permanent magnets for QC1. After beginning of collaboration with FNAL, they would like to use permanent magnets everywhere as possible as they can for cost reduction. For the QC1 with 189 T/m, Sm-Co (1:5) has been chosen for stronger magnetic field(1.5T) and stronger radiation hardness. It consists of 50blocks of 1 inch thickness and its density is 8 g/cm^3. Alignment between the blocks is very important for small higher order harmonics(<10^{-3}). There are three kinds of permanent magnet; (i) ceramic which has low field B=0.38T and large thermal coefficient 0.2%/degree_C, (ii) rare earth cobalt of Sm-Co (1:5 or 2:17) , typically 1.05T and expensive and (iii) Nd-Fe-B, typically B=1.2T and poor radiation hardness. optimal types can be chosen at the BDS. If all the quadrupole magnets (696) are permanent magnets, the cost reduction will be 250M dollars.